1. Field of the Invention
The invention relates to a control apparatus and a control method for air-fuel ratio sensors, and particularly to a technology for heating a sensing element of an air-fuel ratio sensor.
2. Description of the Related Art
A technology is known in which the air-fuel ratio of an internal combustion engine is detected by an air-fuel ratio sensor and the internal combustion engine is controlled to operate at a desired air fuel ratio. Air-fuel ratio sensors detect air-fuel ratios by utilizing the electromotive force of a zirconia element. To activate such an air-fuel ratio sensor, its zirconia element needs to be heated. Therefore, in general, an air-fuel ratio sensor has a heater for heating its zirconia element.
Meanwhile, when the temperature in the exhaust passage in which an air-fuel ratio sensor is provided is low, water droplets exist in the exhaust passage. If the water droplets contact the zirconia element of the air-fuel ratio sensor while it is being heated, it may damage the zirconia element. Therefore, the heating of the zirconia element is carried out when the water droplets are considered to be no longer present in the exhaust passage.
JP-A-2003-227400 describes a control apparatus for controlling the temperature of an air-fuel ratio sensor, which enables early activation of the air-fuel ratio sensor without causing cracking of the sensing element. The temperature control apparatus described in this publication includes a switching portion. In operation, this switching portion keeps a heat-applied portion, which is a portion to which heat of an amount necessary for enabling the early activation of the air-fuel ratio sensor is applied, in a preheated state until the temperature of a protection portion for protecting the sensor element reaches a predetermined value. When the temperature of the protection portion reaches the predetermined value, the switching portion places the heat-applied portion in a heated state in which the heat-applied portion is heated by a larger amount of heat than it is in the preheated state.
According to the temperature control apparatus described in JP-A-2003-227400, the heat-applied portion is preheated until the temperature of the protection portion located close to the sensing element, which temperature thus accurately reflects whether and how much the sensing element is wet, reaches the predetermined value, that is, until the sensing element is no longer wet, and thereafter the heat-applied portion is placed in the heated state where a larger amount of heat is applied to the heat-applied portion than in the preheated state. Thus, it is possible to activate the sensing element at an earlier time while reliably preventing cracking of the sensing element, which may otherwise be caused if the sensing element in wet condition is rapidly heated.
In the mean time, in some internal combustion engines, the temperature of exhaust gas is increased by retarding the ignition timing so as to accelerate the warming-up of a catalyst used to purify exhaust gas. In this case, a protection cover for protecting the sensing element of the air-fuel ratio sensor increases quickly. On the other hand, in the state where the retardation of the ignition timing is prohibited, the increase in the temperature of the protection cover is sluggish. Therefore, if preheating of the air-fuel ratio sensor is continued until the temperature of a protection member (protection cover) for protecting the sensing element reaches a predetermined value as in the case of the temperature control apparatus described in JP-A-2003-227400, the activation of the air-fuel ratio sensor may be delayed unnecessarily.